Determine originating IP address. REMOTE_ADDR is the standard but will fail
if the user is behind a proxy. HTTP_CLIENT_IP and/or HTTP_X_FORWARDED_FOR
are set by proxies so check for these before falling back to REMOTE_ADDR.
HTTP_X_FORWARDED_FOR may be a comma- delimited list in the case of multiple
chained proxies; the first is the originating IP.